Review Question - QID 216857

A 35-year-old male olympic power-lifter presents to your office after sustaining an elbow injury. He reports he was doing curls when he felt a pop and immediate pain in his elbow. On examination, he has distinct weakness with forearm supination and hook test is positive. Given his desire to continue power-lifting, you recommend surgical repair. During surgery, you perform a volar incision and find the interval between the brachioradialis and pronator teres muscles. You place a deep retractor between these muscles to maintain visualization, but immediately notice a flash of bright red blood in the surgical field. What structure has likely been injured?